The above described configuration belongs to boat devices known under as “SWATH” (Small Waterline Area Twin Hull).
These watercrafts are types of vessels that generally have two lateral hulls, namely a Twin Hull, shaped to decrease the area of that part of the hull that is intersected by the surface of the sea during navigation.
Consequently, when a SWATH vehicles is encountered by a surface wave, compared to an equivalent conventional hull (both monohull or catamaran), it is subject to a substantially smaller change in displaced volume of the hull (positive or negative depending on whether the vessel is on the crest or through of the wave) and hence to a smaller buoyant component of the wave exciting force, which is usually the major cause of wave induced ship motions. Therefore, the SWATH vessels have an inherent greater platform stability in waves, which means navigating in relatively high sea states with reduced pitch and heave motions compared to equivalent monohull or catamaran vessels.
In fact, during navigation, the main body of the SWATH is emerged while the two lateral hulls, displacing the majority of the volume, are submerged below the free surface. They are responsible for the increase in stability in sea waves, because they are affected by wave action to a lesser extent than the traditional craft, due to their smaller waterline area.
Consequently, the SWATH watercraft devices appear as a platform that is supported and conducted by two submerged hulls connected to the platform by one or more arms per submerged hull, very slender in the longitudinal direction (fore-aft) or correspondingly thin in the radial direction perpendicular to the longitudinal direction.
Despite a higher seakeeping performance, all the above described SWATH configurations suffer from an increase in resistance to the advance motion and consequently of higher fuel consumption compared to equivalent conventional vessel. Conversely, at a given installed power SWATH vessels can reach a substantially lower maximum speed in calm water, if remaining in the displacement mode (i.e. with their weight supported by the buoyancy force, substantially due to the displaced volume of the submerged hulls).
Moreover, similarly to all displacement and semi-displacement hulls, SWATH vessels are usually not suitable to achieve high relative velocities.
A significant parameter is the Froude number, calculated on the length FnL=Vel/radg (g*L) and on the displaced volume at rest, FnV=Vel/radg (g*V), where Speed=speed of advance, g=gravity acceleration, V=volume displaced by the hull at rest (weight of the device/density of the water), L=total hull length. The operating limit for a displacement/semidisplacement hull is around FnL=0.6/0.8 to about 1.0 (in the case of the SWATH hull form with innovative annular constriction subject matter of this patent), or FnV=2.0/3.0.
When the Froude numbers, or the speed relative to length or displacement, becomes too high, the displacement or semi-displacement hull, having typical hull forms shaped to be efficient in a low speed range, are abandoned and, to ensure an acceptable resistance at high speed, the form of planing hulls are adopted.
However, it is well known that planing hulls suffer in rough seas, reacting with large motions and violent impacts on the incident waves and at high speeds (corresponding to FnL>1.0) and becoming practically inoperable in sea waves (even moderate sea state two or three), due to the high accelerations experienced by the vessel and the high impact forces on the waves.
Therefore, there is an unsatisfied need to create a watercraft device that, at the same time, has very good sea-keeping characteristics (reduced motions in rough seas) and reduced resistance to advance motion in two speed regimes, one corresponding to the displacement mode and the other typical of planing hulls, significantly increasing the upper limit of operation of the latter, in terms of maximum speed (over 80 knots) in a relatively low sea state.
This allows reaching high speeds in navigation, even higher than traditional monohull and catamarans vessels, maintaining, at least at low speeds, a level of stability in waves comparable to that of SWATH devices according to the state of the art.